Abstract: Bose--Einstein correlations between identical pions close in phase-space is thought to be responsible for the observed distortion in mass spectra of non-identical pions. For example in the decays \rho 0 \rightarrow \pi + \pi - and \rho \pm \rightarrow \pi \pm \pi 0, such distortions are a residual effect where the pions from the \rho decay interact with other identical pions that are close in phase-space. Such interactions can be significant in, for example, hadronic decays of Z bosons where pion multiplicities are high, and resonances such as \rho mesons decay with a very short lifetime thereby creating pions that are close to prompt pions created directly. We present the S{ö}ding model and show that it has been used successfully to model distortions in \pi \pm \pi 0 mass spectra in hadronic Z decays recorded by ALEPH.